The theory of the motions of atoms and molecules in a fluid provides the key to understanding thermodynamics, yet many students find difficulty in evaluating and using the basic expressions of kinetic theory. This book is designed to lead such students through the fundamental assumptions underlying the theory and to build up a clear physical understanding of the distribution of molecular speeds within a gas. The bulk of the text outlines the relationship between molecular motions and the macroscopic properties of gases (including the equation of state for a perfect gas and for a van der Waals gas), and derives a variety of transport coefficients and thermodynamics relations.